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Atomic
structure
Francine
Taylor-Campbell, Contributor
We
will continue to look at the structure
of atoms in this lesson.
MAIN
POINTS
-
Electrons are arranged in shells
around the nucleus.
- Each
shell or energy level can hold a
fixed number of electrons.
When
a shell is filled electrons will
enter the next shell with the lowest
energy.
- Isotopes
are atoms of the same element with
the same atomic number but different
mass numbers.
- Elements
are arranged in the periodic table
in order of increasing atomic (proton)
number.
Last
week, we found out that protons and
neutrons are present in the nucleus
while the electrons orbit the nucleus.
These electrons are not stationary
but continue to move in different
energy levels around the nucleus.
The arrangement of electrons in the
shells or energy levels is governed
by certain rules or guidelines.
1.
Electrons start to fill up the shells
of lowest energy first; i.e., the
closest one to the nucleus.
2.
The first shell (K shell) can hold
a maximum of two electrons.
3.
The next shell (L shell) can hold
a maximum of eight electrons.
4.
The third shell (M shell) holds eight
electrons for the first 20 elements
but can hold up to 18 electrons.
Let's
take the element Boron for example:
Example
1
115B
Boron has a mass number of 11 and
an atomic number of 5.
This
means it also has 5 electrons. Using
the guidelines B can have 2 electrons
in the first shell which leaves 3
electrons in the second shell. The
electronic configuration (EC) of B
= 2:3
Example
2
Sodium
- 2311Na
EC
of Na = 2:8:1
Remember
the second shell can hold no more
than eight so the other electron must
go into the third shell.
Try
the following. Write the electronic
arrangement of the following elements;
He, C, Al and K.
There
are some elements that exist as isotopes.
For
example, 3517Cl
and 3717Cl .
Notice
that both elements have the chlorine
symbol and have the same atomic number
but their mass numbers are different.
Remember mass number = P + N
If
#protons = 17 then for 35Cl
- the # neutrons = 35 -17
= 18 while for 37Cl - #neutrons =
37-17 = 20 neutrons.
These
two chlorine atoms will have the same
chemical properties, as their protons/electrons
will be the same. It is the electrons
in atoms that react and influence
the chemical properties.
Let's
look at another example, oxygen-16
and oxygen-18.
168O
and 188O.
Both
atoms have 8 electrons but one atom
has 8 neutrons while the other has
10.
Atoms
of some isotopes are unstable and
can split to form smaller atoms. When
this occurs their nucleus splits and
radiation is released. These isotopes
are called radioisotopes or radioactive
isotopes. Radioactive particles released
during splitting may be:
1.
alpha particle which is a helium ion
(helium atom that has lost its electrons
He2+)
2.
Beta particles which are electrons
(charge of -1)
3.
Gamma particles are high-energy electromagnetic
radiation.
Radioisotopes
are used in medicine and in industry.
1.
In the treatment of cancer, gamma
radiation from cobalt-60 is used to
destroy cancerous cells.
2.
In biological research, iodine-131
and carbon-14 are used as tracers.
Iodine -131 is given to patients with
defective thyroid glands while C-14
is used in the dating of plant and
animal remains.
3.
Radioactive isotopes, like uranium-235,
are used to generate electricity.
When a nucleus of this atom splits,
a chain reaction is set up which releases
vast amounts of heat energy. If this
reaction is not controlled this can
lead to an atomic bomb.
Elements
are arranged in the periodic table
in groups based on the number of electrons
in the outer shell. So all elements
with 1 electron in the outer shell
are in group 1 and those with 2 electrons
are in group 2. Elements with a filled
shell (2 or 8 electrons) are in group
8 or O. The arrangement of elements
in groups influences reactivity and
the types of reactions they will participate
in.
We
will examine this issue in more depth
in later lessons.
Francine
Taylor-Campbell teaches at Jamaica
College. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
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